In medical procedures it is sometimes the case that MRI imaging or fluoroscopic imaging cannot provide all the necessary information needed about the location of a catheter, guidewire, electrode or endoscope inside a patient. For example, in order to accurately navigate medical devices in a patient's body, it is important to be able to accurately localize, i.e., determine the position and/or orientation of, the device. Various methods have been used for localizing medical devices in the body, the processing of x-ray images, electric potential measurement, ultrasonic measurement, and magnetic measurements. A useful method for determining the tip position and orientation of these devices has employed low frequency electromagnetic communication between a transmitter or receiver attached to the tip and a corresponding receiver or transmitter fixed at a position near but outside the patient.
In typical magnetic localization methods, the medical device has at least one receiver, and a transmitter external to the body has a plurality of coils each emitting a different frequency. Sometimes the transmitting coils are separated by significant distances to provide a useful parallax in the vector analysis of the signals from the receiving sensors (which are preferably orthogonal, but at least having orientations which will provide a basis set spanning the space of three dimensions).
There are numerous examples of magnetic localization, including in U.S. Pat. Nos. 5,694,945, 5,846,198, 5,738,096, 5,713,946, 5,833,608, 5,568,809, 5,840,025, 5,729,129, 5,718,241, 5,727,553, 5,391,199, 5,443,489, 5,558,091, 5,480,422, 5,546,951, 5,752,513, 6,092,928, 5,391,199, 5,840,025, U.S. patent application Ser. No. 09/809523, filed Mar. 15, 2001, and published Nov. 29, 2001, as No. 20010045826, and PCT Application No. PC/US01/08389, filed Mar. 16, 2001, and published Sep. 20, 2001, as WO 01/69594 A1, and PCT/GB/01429, published Nov. 16, 2000, as WO 00/68637, the disclosures of all of which are incorporated herein by reference.
While magnetic localization offers a number of advantages, it has suffered from one significant disadvantage—the presence of metal, such as from other medical or imaging equipment, near the operating region impairs the accuracy of magnetic localization. While it is possible through calibration to account for some metal in the operating region, any movement of the metal in the operating region generally requires an extensive and time consuming recalibration to accurately localize the magnetic device.